BUILDING STONES. 641 



riety in Miller's Creek, southeast of Lone Grove, which may be worthy of 

 investigation. In Yoakum Hollow similar outcrops occur, and a variety of 

 this class is favorably exposed in the Cat Mountains. Four miles north of east 

 from Enchanted Rock, and in much of the surrounding area, some very choice 

 material of the kind may be mined, as well as southward in the northern 

 portion of Gillespie County. 



THE COMPRESSED GRANITES. 



There are many of the Archean granites which appear as if tightly wedged in between the 

 adjoining strata, although the result is perhaps more often due to the expansion produced by 

 the crystallization of the magma. For this last reason the material is frequently an aggrega- 

 tion of large interlocking crystals, without coherence enough to render the rock useful for 

 any purpose where stress is anticipated. Occasionally some very fine crystals of orthoclase 

 and microcline of immense size, or tough masses of graphic granite admitting of a high 

 polish, occur in similar situations. 



There is little to add this year to the above statement taken from page 366 

 of my former Report. It is not probable that the compressed granites will 

 ever cut any great figure in the production of our region, if surface indica- 

 tions prove trustworthy as criteria for deeper workings, which can hardly 

 admit of doubt. 



THE BLOCK GRANITES. 



Again I have to quote from my Preliminary Report for 1889, as the desig- 

 nation of this class can not be materially improved after longer study, and 

 the edition of the first Report was limited: 



This name is proposed for a class of granites which occur in various parts of the Central 

 Mineral Region, and which may represent more than one uplift. They are not yet fully 

 understood, and there is here no intention of putting them together except for the conveni- 

 ence of treatment. They shade off into dimension granites upon the one hand and into fissile 

 granites upon the other, in a general sense, and yet they are distinctive, more by reason of 

 their texture than their structure. At the same time it is possible that some of the friable 

 granites, or "rotten granites," may be only the block granites undergoing a process of decay. 

 But a commercial classification must necessarily deal with present conditions, and upon that 

 principle this class name is justifiable. I would include under it all those granites which 

 are solid enough to withstand sharp blows from a single-hand hammer without shattering, 

 except along the line of impact, so that they may be readily broken into blocks. But, in 

 addition, they must have definite joint or lamination planes, usually induced by the mode of 

 arrangement of the large crystals of feldspar. Any of the other types, howsoever well they 

 may be broken into shape by careful trimming, do not at once assume the block form char- 

 acteristic of this class. Thus restricted, there is considerable variation in the solidity of the 

 blocks, even when they are not undergoing rapid disintegration. This difference arises from 

 the irregularity of coherence among the feldspar crystals. Some of the block granites are 

 less ferruginous and paler in color than the classes previously described, but when there is an 

 abundance of siliceous cement this loss of iron does not appear to weaken the combination. 



